The implementation of increasingly stringent regulations on emissions released by internal combustion engines has encouraged development of technological advances in engine configuration. Such improvements have assisted in a reduction in the release of combustion products and other byproducts to the atmosphere while maintaining and often improving engine performance. For example, by incorporating a turbocharger, a vehicle's efficiency and power output may be increased.
Turbochargers forcibly induct extra air into combustion chambers of the engine, resulting in ignition of additional air to maintain combustion stoichiometry, thus producing more power than achieved by delivery of intake air at ambient pressure. Turbochargers typically comprise a compressor driven by a turbine via a connecting drive shaft. The rotation of the turbine is often powered by rapidly expanding exhaust gas generated at the combustion chambers, a portion of which is channeled to the turbine to drive the boosting of intake air. The spent exhaust gas is then treated by an after treatment device, such as a catalytic converter, to remove emissions before releasing the exhaust gas to the environment.
A portion of the exhaust gas flow may also be diverted from an engine exhaust to an engine intake manifold in engines configured with exhaust gas recirculation systems (EGR). The recycling of the exhaust gas provides a desired engine dilution which reduces engine knock, in-cylinder heat losses, throttling losses, as well as NOx emissions. In turbocharged diesel engines, EGR may significantly reduce the formation of NOx by lowering the peak combustion temperature at the cylinders. Since the exhaust gas flow is divided between the EGR system and turbine rotation, the turbocharger is often a variable geometry turbocharger (VGT). The use of the VGT allows for control over the flow of exhaust gas into the turbine nozzle by varying the geometry of the nozzle, thereby controlling the amount of exhaust gas available for EGR as well as engine dilution.
A high pressure EGR (HP-EGR) system relies on a negative pressure differential (e.g., exhaust pressure greater than intake pressure) at the engine during low engine speed operations in order to promote EGR flow. When EGR gas displaces some of the boosted ambient air normally ingested into the engine, increased boosting of intake air is induced to maintain appropriate air/fuel ratios for combustion efficiency. To accommodate higher boost levels, the VGT vanes may be adjusted to a more closed position. The velocity of the gas flow increases, providing more power to the turbine. By reducing the area of flow through the VGT vanes, pressure upstream of the turbine is increased, promoting EGR flow.
Thus, the EGR flow may be controlled by the vane position of the VGT. The vanes may be adjusted to reduce the level of NOx in the exhaust gas to a desired level and results in the delivery of charge air in amounts sufficient to maintain target air/fuel ratios in the engine. One example of a boosted engine adapted with a VGT and an EGR system is shown by Rimnac in U.S. Pat. No. 6,460,522. Therein, a method and apparatus for controlling EGR is disclosed including the VGT in communication with the EGR system and a first rotary electric actuator controlling the position of the VGT. A second rotary electric actuator is operatively connected to an EGR valve for further control of the EGR flow.
However, the inventors herein have recognized potential issues with such systems. As one example, VGTs and turbochargers in general have become increasingly efficient and less turbine power is needed to meet boost demand. The higher efficiency of the VGT results in the maintaining of the VGT vanes in a more open position where the gaps between the vanes are widened in comparison to the VGT vane position of a less efficient VGT. The more open position of the VGT vanes allows increased air flow through the turbine nozzle, reducing the pressure in the exhaust passage upstream of the turbine. Consequently, the pressure differential driving EGR flow is diminished, adversely affecting the beneficial effects of the EGR system as described above. Attempts to reinforce the negative pressure differential using engine throttling to increase vacuum at the intake manifold or incorporating a pump in the EGR passage leads to poor fuel economy of the vehicle.
In one example, the issues described above may be addressed by a method for decreasing compressor efficiency and increasing a pressure differential between an exhaust system upstream of a turbine and an intake system downstream of a compressor and, responsive to increasing the pressure differential, opening an exhaust gas recirculation (EGR) valve to flow exhaust gas from the exhaust system to the intake system. In this way, EGR flow may be maintained without throttling the engine, at least during some conditions, thus avoiding additional fuel consumption.
As one example, a wide range active compressor (WRAC) may be configured with an active casing treatment including a surge slot and a choke slot. The surge slot and choke slot may be alternatively opened and closed in response to driving routines and engine speeds. During vehicle operation at low engine speeds, the surge slot is usually maintained open, allowing recirculation of air from an impeller to an intake passage of the compressor inlet. The recirculating flow alleviates pressure accumulation at an outlet end of the compressor, increasing compressor efficiency. In contrast, by opening the choke slot during low engine speeds, pressure buildup at the compressor outlet slows the rotation of the turbine, allowing pressure upstream of the turbine to increase. Compressor efficiency is reduced and EGR flow is improved.
Alternatively, the WRAC may also comprise a variable inlet device that regulates flow into the compressor inlet by varying the inlet diameter. During compressor operation under light loads, e.g., low mass flow, the variable inlet device is often adjusted to decrease the diameter of the compressor inlet to restrict flow and prevent compressor surge. By opening the variable inlet device and increasing the diameter of the compressor inlet, compressor efficiency may also be reduced, enhancing EGR flow.
By configuring a WRAC with an active casing treatment or variable inlet device, the compressor efficiency may be modulated to increase the pressure gradient in the engine for EGR flow. In this way, the pressure differential between the exhaust manifold and intake manifold may be controlled by the adjustment of compressor inlet air flow by the variable inlet device to decrease boost pressure or by the direction of additional flow through the compressor inlet as determined by the positioning of the active casing treatment.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.